The present disclosure relates generally to test systems for fiber optic cable and methods of operating the same and, more particularly, to differential mode delay (DMD) test systems for multi-mode fiber optical cable and methods of operating the same.
When an optical source is directly coupled to a multi-mode fiber DMD may occur. DMD refers to a phenomenon where a single light pulse excites multiple modes within a multi-mode fiber that propagate at different speeds through the multi-mode fiber causing the different modes to be spread in time. Thus, when DMD occurs, a single pulse launched into a multi-mode fiber may turn into multiple independent pulses, which may interfere with other pulses in the data stream and, thus, makes it difficult to recover the data encoded.
A test system may be used to evaluate the DMD characteristics of a multi-mode fiber. Such systems typically launch optical test pulses into the core of a multi-mode fiber and evaluate the DMD associated with these test pulses. A DMD test system may use a single-mode fiber to launch the test pulses into the core of the multi-mode fiber. It may be difficult, however, to optically center the single-mode fiber with the multi-mode fiber to launch the DMD test pulses into the core of the multi-mode fiber, as a single-mode fiber may have a core diameter in the range of about 8-10 μm and a multi-mode fiber may have a core diameter in a range of about 50-62.5 μm. Single-mode fibers used to carry light pulses having a wavelength of around 850 nm may have an effective optical mode field core diameter of 5 μm while multi-mode fibers used to carry 850 nm light pulses may have a core diameter of 50 μm. One technique for centering a single-mode fiber with a multi-mode fiber is to attempt to center the two fibers geometrically where a smaller diameter core single-mode fiber is imaged into the larger diameter core multi-mode fiber. Unfortunately, geometric centering may involve expensive and relatively large vision equipment. Another technique for centering a single-mode fiber with a multi-mode fiber is based on motion control and coupled power optimization, which may require a relatively expensive and large feedback stabilization system. Conventional centering techniques, therefore, may be difficult to use in a field environment where portability is desired as many of the component parts of the feedback system may not be available in smaller sizes. Moreover, geometric centering may often be inaccurate as the geometric center of a multi-mode fiber may not be the optical center of the multi-mode fiber.
FIGS. 1A and 1B illustrate a conventional technique for aligning two fibers of the same type, i.e., two single-mode fibers or two multi-mode fibers, based on the coupling of optical power between the two fibers. As shown in FIG. 1A, one of the two single-mode or multi-mode fibers is fixed while the other single-mode or multi-mode fiber is moved along the X and Y axes while the optical power that is coupled between the two fibers is measured. The graph shown in FIG. 1B indicates that the two single-mode fibers are closest to being centered when the optical power (P) coupled between the two fibers is at the maximum along both the X and Y axes. The centering system determines the X-axis position where the power coupling is maximum and then performs the same procedure along a Y-axis, perpendicular to the X-axis (not shown). After determining the position where the coupled power (P) is at a maximum level based on relative positioning along both the X and Y axes, the two fibers are generally substantially centered. Additional iterations may be performed to increase the accuracy of the centering.
But, as described above, a multi-mode fiber has a core region having a diameter that may be approximately ten times that of the effective optical mode field of a single-mode fiber. Therefore, as shown in FIGS. 2A and 2B, when one of the single-mode fiber and the multi-mode fiber is fixed and the other is moved along an axis the optical power that is coupled between the two fibers may not have a distinct peak, but instead may have a relatively broad range where the coupled power is maximized due to the smaller core diameter of the single-mode fiber. As shown in FIG. 2C, when the single-mode fiber is moved along with the X-axis, two positions can be identified where the power coupling drops below a maximum level. These two positions A1 and B1 identify the boundary of the cladding surrounding the core of the multi-mode fiber and are shown connected by the dashed line in FIG. 2C. The dashed line illustrates the movement of the single-mode fiber relative to the multi-mode fiber in evaluating the coupling of optical power between the single-mode fiber and the multi-mode fiber. The distance between positions A1 and B1 can be divided by two to find an approximate center O1 along the X-axis. The process can then be repeated by moving the single-mode fiber along the Y-axis, which is perpendicular to the X-axis and determining the two positions where the power coupling drops below a maximum level. These two positions C and D identify the cladding boundary of the multi-mode fiber and are shown connected by the dotted line in FIG. 2C. The dotted line illustrates the movement of the single-mode fiber relative to the multi-mode fiber in evaluating the coupling of optical power between the single-mode fiber and the multi-mode fiber. The distance between positions C and D can be divided by two to find an approximate center O2 along the Y-axis. Theoretically, moving the single-mode fiber to the position based on dividing the distance between C and D by two would result in the single-mode fiber being optically centered with the multi-mode fiber. Unfortunately, the determination where the coupled optical power falls below the maximum level, which is used to identify the boundary positions A1, B1, C, and D, may be imprecise. Thus, the above-described procedure for determining the optical center along the X and Y axes may be repeated for several iterations until the single-mode fiber and the multi-mode fiber are optically centered with a desired precision. For example, single-mode fiber may be moved along the X-axis to determine two positions A2 and B2 where the power coupling drops below a maximum level. These two positions are shown connected by the solid line in FIG. 2C and approximately run through the point O2 indicating that O2 is the optical center of the multi-mode fiber. The determination of the optical center O2 can be summarized by the following relationships, where X-Y is read as the distance between X and Y:A1−O1=O1−B1=½(A1−B1)C−O2=O2−D=½(C−D)A2−O2=O2−B2=½(A2−B2)A2−O2=O2−B2=C−O2=O2−D=R, where R is the radius of the multi-mode fiber core.